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1、Passage7 Noseshaveitprettyhard.Boxersfattenthem.Doctorsrearrangethem.Peoplemakejokesabouttheirunflatteringcharacteristics.Worstofall,whenitcomestosmell,noonereallyunderstandsthem. Despitethenose'sconspicuouspresence,itsworkingsaresubtle.Smell,orolfactionisachemosense,relyingonspecial
2、izedinteractionsbetweenchemicalsandnerveendings.Whenarose,forexample,issniffed,odormoleculesarecarriedbytherisingairstreamtothetopofthenasalcavity,justbehindthebridgeofthenose,wherethetipsofthetensofmillionsofolfactorynervecellsareclusteredinthemucouslining.Themoleculessomehowtriggerth
3、enerveendings,whichcarrythemessagetotheolfactorylobesofthebrain.Becausesmellinformationthentravelstootherregionsofthebrain,thescentofarosecanelicitnotonlyapleasurablesensationbutemotionsandmemoriesaswell. Thoughjusthowodorsstimulatethenervesisunknown,scientistsdoknowthatoursenseofsmel
4、lissurprisinglykeen,capableofdistinguishinguptotensofthousandsofchemicalodors.Thelaboratorytaskofisolatingthecomponentsofanodorisfarfromsimple.Tobaccosmoke,forexample,ismadeupofseveralthousanddifferentchemicals.Moreover,smellresearchersmustgrapplewiththeproblemofwhattocallthedifferento
5、dorsthatthenosedetects.Peoplegenerallyrefertosmellsbytheirsourcesofassociations.Descriptionssuchas"likeawetdog"or"likemyelementaryschool"mayconveyperceptionsbutarevastlyinadequateforlabelingthechemistryinvolved. Tofurthercomplicateresearch,olfactionisconnectedtoothersensations.Besides
6、olfactorynerves,thenasalcavitycontainspain-sensitivenervesthatperceivesensationssuchasthekickinammoniaoftheburninginchilipeppers.Smellalsointerwineswithtastetocreateflavor.Acoffeedrinkerholdinghisnosewhilesippingwouldtasteonlythebitterinhisbrew,fortastereceptorsgenerallyappearlimitedto
7、bitter,salty,sourandsweet.Thesenseofsmellistenthousandtimesmoresensitivethantasteandmakessubtledistinctionsamonglemon,chocolate,andmanymoreflavors. Sohowdoesthenosemanagethissophisticateddiscrimination?Lakeofevidencehasn'tkeptscientistsfromspeculating.Oneideaisthateveryodormoleculev
